1. Field of the Invention
The present invention relates to a screw backpressure controlling method in a plasticizing process of an electric injection molding machine.
2. Description of the Related Art
In a conventional screw backpressure controlling method, a linear movement of a screw that retreats due to a melted resin pressure generated by plasticization is converted into a rotary movement and, then, a rotary force and a rotation frequency of a servomotor rotated by the rotary movement is controlled. The rotary force may be controlled by feedback controlling the servo motor so that the screw backpressure obtained by measuring the melted material pressure or the screw retreating force electrically conforms to a set value of the screw backpressure (for example, see Japanese Unexamined Patent Publication (Kokai) No.61-31221). Further, the rotation frequency may be feedback controlled so that the backpressure conforms to a set pressure by detecting a pressure applied to the resin, determining a difference between a set backpressure and the detected pressure, and outputting a movement command based on said difference between the set backpressure and the detected pressure to a servo circuit that drives and controls the servo motor (for example, see Japanese Unexamined Patent Publication (Kokai) No.04-249129).
In yet another technique, a moving speed of the screw is feedback controlled so that the detected resin pressure is equal to the set backpressure, wherein a speed setting command value for a servo motor is varied according to an override value to allow the screw to move backward continuously and smoothly (for example, Japanese Unexamined Patent Publication (Kokai) No.02-120020).
However, because all of the conventional screw backpressure controlling method described in the above Japanese Unexamined Patent Publications are based on feedback control, it is inevitably needed to set constants such as a proportional gain P, an integral gain I and a derivative gain D. In order to improve control accuracy, these PID constants are desired to be set as high as possible. However, the injection molding is applied to a great many types of resin materials, flowability and melting properties of which vary in a wide range. Therefore, if the PID constants are set on the assumption of an average resin material, there will be problems in that the gain is too low to ensure the control accuracy in one resin material or the gain is so high that the control becomes unstable and oscillates.
The present invention is made to solve the problems described above with regard to the prior art and provides a screw backpressure controlling method in an electric injection molding machine that can achieve controlling accuracy comparable to the one in feedback control with simpler configuration, even though it is not the feedback control method but an ON-OFF control method.
Thus, according to claim 1 of the present invention, there is provided a screw backpressure controlling method in an electric injection molding machine comprising: a sensor for detecting a screw backpressure as an electric signal; servo motors for driving a screw forward and backward; a motor for rotating the screw to plasticize a material; and a controller for controlling said servo motors and said motor,
wherein, in a plasticizing process in which said motor is rotated, said controller performs control by repeating a first process in which the rotation of said servo motors is controlled by a first retreating speed command value to make the screw retreat at a low speed when a screw backpressure signal is smaller than a backpressure set value and a second process in which the rotation of said servo motors is controlled by a second retreating speed command value to make the screw retreat at a high speed when the screw backpressure signal is larger than the backpressure set value.
According to one embodiment of the present invention, in the screw backpressure controlling method in the electric injection molding machine when said first process continues longer than a predetermined monitoring time, the first process is performed according to a first computed value that is obtained by subtracting an additional speed command value from said first retreating speed command value and the second process is performed according to a second computed value that is obtained by adding a speed difference to said first computed value, wherein the speed difference is, in turn, obtained by subtracting said first retreating speed command value from said second retreating speed command value.
According to one embodiment of the present invention, in the screw backpressure controlling method in the electric injection molding machine when said second process continues longer than a predetermined monitoring time, the second process is performed according to a first computed value that is obtained by adding an additional speed command value to said second retreating speed command value and the first process is performed according to a second computed value that is obtained by subtracting a speed difference from said first computed value, wherein the speed difference is, in turn, obtained by subtracting said first retreating speed command value from said second retreating speed command value.
According to one embodiment one embodiment of the present invention, in the screw backpressure controlling method in the electric injection molding machine, the retreating speed command value is switched from said first process to said second process or from said second process to said first process according to a predetermined temporal variation.
According to one embodiment of the present invention, in the screw backpressure controlling method in the electric injection molding machine, said monitoring time is changed according to how much said screw backpressure signal is deviated from said backpressure set value.
According to one embodiment of the present invention, in the screw backpressure controlling method in the electric injection molding machine, the monitoring time when said screw backpressure signal is deviated from said backpressure set value significantly is set to be shorter than the one when said screw backpressure signal is deviated slightly from said backpressure set value.